The need to identify quantitatively as well as qualitatively different materials on surfaces, in solutions, or in gas mixtures is widely recognized. Identification of molecules, molecular fragments, atomic species, plasma swept or occupied environmental conditions such as temperature (thermal and quantum), ionizing radiative flux, pressure, flow, optical properties, as well as density gradients at a large range of concentrations is of particular interest.
The earliest historical chemical identifications were done with simple dye indicators that provided a color change to indicate a property, such as Ph. Other physical property observations such as color, melting point, miscibility, density, or the appearance of combustion, i.e. visible emissions of fire, provided indications of presence of certain substances or elements. However, these methods are very imprecise and provide minimal information about the actual concentrations of the target species, although fire did provide among the first known controlled light sources; which also indicated the presence of liquid water by extinguishment of the fire light observed with the eye.
Flame colors were later recognized to be an effective indicator of multiple atomic species and even later became relatively quantitative in flame spectroscopy. These flame colors have been applied for entertainment in fireworks for a long time. Relatively large quantities of target species are required for reliable detection due to the very low and dispersed energies of the emitting light and the limited time window for emissions.
Many attempts have been made by inventors to improve detection and emission efficiency, throughput, costs, and lifetime of flame ionization detector designs. Basic problems with current designs using generalized thermal ionization is a lack of directionality of emissions coupled with the inconsistent rates and amounts of completely ionized sample material. Edge effects of reaction chambers, as well as hot and cold spots in combustion processes, contribute to incomplete ionization, distributed emissions, and distributions of effective thermodynamic temperatures within the reaction chamber.
Another form of ionization based detection is Fluorescence spectroscopy, also known as fluorometry or spectrofluorometry. In this technique, light photons are used to instigate boosting of molecular electrons into higher energy levels, as in pumped molecular lasers. These electrons then drop back into lower energy levels while releasing omnidirectional photons of light. These photons are then analyzed for energy corresponding to the energy level change in the molecule. This energy can then be related to the originating molecule type, emission structure, or species.
A major aspect of fluorometry is that there are many vibrational energy levels of target molecules and/or atoms, causing energy of emitted photons therefrom to take corresponding energy values depending on the quantum vibration energy states of the various target species. This variation in the emitted photon energies can be used to elucidate the vibrational, rotational, and vibronic coupling energy separations when there are numerous photons to analyze at all energy levels, with the analysis used for identifying particular molecules, molecular structure, or shape.
Lasers or spectrum sampled broad band light sources, with variable bandwidths of photons, can excite specific and predictable molecular transitions. The advantage to the dispersion or filter separated broadband sources is that they are easily scanned over known wavelengths to create various stimulation profiled emissions. Lasers are typically very narrow energy band photon emitters, although some lasers, such as dye lasers, are tunable to produce photons of a desired wavelength, and thus a desired energy level. However, in any of these methods, difficulties are encountered in tuning or scanning the input stimulation photons to a sample across a waveband and then cross scanning an output emission spectrum analysis from the sample for each input wavelength. Not only do such methods lead to very large, noisy integrated data sets, long periods of time are required to complete an analysis, particularly with a very dilute analyte. Further; the analyte itself may be undesirably modified by the stimulating radiation.
A recognition of these limitations and the requirements of large scale industry has led to several notable improvements, such as multiple band illumination contemporary with multiple excitation regions, multiple detectors, and/or multiple dispersion elements. While these improvements speed up the analysis, they still generate large data sets required to be processed and also require large integration times for dilute analyte analysis.
Lasers, photodiodes, and lamps, particularly xenon arcs and mercury-vapor lamps, are among photonic light sources typically used for sample stimulation. Each of these sources present difficulties such as limited lifetimes, a continuously degrading operational output, high operational temperatures, wide emission bands and other factors that present complexities in industrial practice. Lasers and photodiodes have a very narrow bandwidth output, which may be advantageous in some instances and a disadvantage in other instances. Further, while lasers and photodiodes may be tunable to some extent, lasers and photodiodes capable of relatively large excursions of wavelength are not available. In addition, relatively inexpensive sources of ultraviolet A to vacuum ultraviolet photonic radiation, which would be useful in some embodiments of the instant invention used in a vacuum, are also not available.
These aforementioned methods all use inefficient electrical energy conversion into photonic energy for stimulating molecular emissions, with various schemes for analyzing the resulting emissions. The disadvantages of driving the entire process with photonic energy are that:
(a) The inherent losses due to entropy make these approaches inherently inefficient uses of power. The fact that most of the stimulating energy is lost to heating the sample, heating the chamber, or leaving the environment cause these methods to be systematically inefficient. This waste of energy is particularly unfortunate where portable applications are needed, or low analyte concentrations are available, which may require long integration times.(b) The material and financial costs of these approaches are very high due to esoteric material requirements for light sources, complex fabrication requirements, short lifetime of stimulating elements, hazardous and esoteric material use, and other manufacturing and material limitations.(c) The reaction chambers must be made of wide band transparent materials, which typically are expensive consumables.(d) The photonic nature of the sample stimulation also suffers from Rayleigh and Raman scattering. Like the signal emissions, this undesirable scattering of the stimulation and generally longer Raman wavelengths is omnidirectional. This causes excess photon counts and mistaken counts due to Raman shifted wavelengths overlapping true electronic transition emissions on the detectors.(e) The ideal energy efficiency for light emission is decremented by both the emission process itself as well as inefficiencies of the production of the stimulating radiation. This makes the overall system efficiencies very poor for photon stimulated photon emission use as a sensible light source.
Electron impact ionization is a technique in which high kinetic energy electrons pass near or into target analyte molecules, with the electrons inducing ionization, excitation, and/or some or complete molecular fragmentation, depending on intensity and energy levels of the electron flux. Near molecular collisions by highly energetic electrons causes large fluctuations in the electric and relative magnetic field around the neutral molecules and induces ionization and fragmentation. This process is used to prepare samples for induction into mass spectrometry analysis. As is currently practiced, electron ionization is not viable for inducing characteristic photonic emission from affected analyte molecules and atoms.
Laser induced breakdown spectroscopy (LIBS) is a method in which short, narrow band intense laser light irradiance of a sample surface ablates a small amount of sample material to a thermal plasma. This plasma then emits relaxation spectra that is analyzed for atomic emission content with or without crossed secondary lasers. This technique depends on complete molecular disintegration and generalized atomic ionization so that only atomic emissions are utilized. Reproducibility of the results is sometimes limited due to variation in the laser coupling and resultant plasma. Coupling LIBS capabilities with free high energy ion or electron impact plasma maintenance or subsequent plasma process is envisioned as an analyte induction or preparation method to interface with this electric discharge invention.